Silencing network



Ap r16, 1940. N. P. cAsE SILENCING NETWORK Filed Feb. ll, 1958 INVENToR L B son P GAS ATTORNEY UNITED STATES PATENT OFFICE SILENCING NETWORK Nelson P. Case, Great Neck, N. Y., assignor to Hazeltine Corporation, a corporation of Delaware Application Fetnmry 11, 193s, serial No. 139,992

REISSUE'D 4 Claims.

This invention relates generally to a silencing network for modulated-carrier signal receivers and particularly to a silencing network to be utilized in a receiver including a tuning indicator 5 such as the cathode-ray tuning eye. Also, the

(Cl. Z50-20) volts above ground. As there is a relatively high amplification in the signal-translating channel of the receiver succeeding the point at which these alternating potentials are introduced, dimculty is experienced in preventing a low-frequency invention is particularly suitable for use in a hum in the loud-speaker of the receiver. If the 5 receiver in which a plurality of vacuum tubes same type of silencing network is utilized in a. in the signal-translating channel of the receiver receiver having the cathode of the rst audiohave. their cathodes directly grounded. frequency amplifier grounded, it follows that the m It is desirable to incorporate in certain radio cathode of the amplifier which ampliiies the aureceivers both a visual tuning indicator and an tomatic volume control potential is several volts arrangement for'silencing the receiver during the below ground potential. Thus, in the convenintervals when the receiver is mistuned, torpretional receiver circuit, the automatic amplicavent distorted signals and noise from being retion control source must provide potentials more 1g produced. Many such silencing networks are negative than the cathode of the amplier tube 15 known, the use oi which is attended by certain which provides the silencing potential. Further, disadvantages. One of the disadvantages of the the cathodes of the tubes of the signal-translatprior art silencing networks is that they have ing channel of the receiver preceding the point generally utilized at least one additional vacuum from which the automatic amplication control tube or its equivalent. potential is derived must also be more negatively 20 There is no potential available in the convenbiased, a condition which is obviously undesirtional radio receiver suitable to be utilized as a able. silencing voltage for the receiver without fur- It is an object of the invention to provide a ther amplication. Furthermore, the silencing simple and economical silencing arrangement for w means previously utilized have generally been a modulated-carrier signal receiver comprising a more responsive to strong signalsl than to weak cathode-ray tuning indicator. ones, the receiver being silenced for a lesser de- It is a further object of the invention to progree of mistuning when weak signals are being vide a signal receiver including a silencing netreceived. work having a substantially uniform response to 3@ One silencing network which has been usedreceived signals of varying intensity. 50 commercially comprises a first audio-frequency It is another object of the invention to proamplfler he-Vng a nolnlel Cathode Potential SeV- vide a sharply selective silencing network for a eral Volts above ground and e silencing network signal receiver to ensure silencing except when coupled to the signal channel of the receiver at the receiver is precisely tuned.

p a Point preceding the flrSi audio-frequency an- In accordance with the present invention,

Pliner. This silencing network comprises a dithere is provided a modulated-carrier signal rerect Current amplier having a grounded oathceiver, which may be of the superheterodyne ode for amplifying the automatic amplifoetype, including an automatic amplification contion control Voltage 0f the receiver to PIotrol system for maintaining the signal ampli- 4p vide a silencing Voltage for lthe lSt audio-fletude at a given point in the receiver, for exam- 40 fluency amplifier tube- The silencing voltage ple, at a point in the intermediate-frequency thus Provided iS applied Positively to the Control channel, within relatively narrow limits. A limgrid of the first audio-frequency ampliner, but iter-amplifier is coupled to this point for derivis of such value that the bias of the control grid ing a signal substantially free from residual w is not raised above its normal cutoff value exvariations is amplitude, the limiter-amplifier incept when the receiver is properly tuned. Howcluding a sharply-selective circuit tuned to the ever, with the arrangement just desecribed in a signal-carrier frequency and following the point receiver having an alternating currentpower supat which limiting occurs. 'I'here is also proply, difficulties are encountered in that audio-frevided means including a rectifier coupled to the 5o quency currents are introduced in the audio-frelimiter-amplifier for deriving a potential to 50 quency channel of the receiver due to the fact that an appreciable potential exists between the cathode heater of the iirst audio-frequency ampliiier, which is usually at ground potential, and its cathode which, as stated above, is several silence the receiver. In a preferred embodiment of the invention, the receiver includes a cathoderay tuning indicator which is also utilized for amplifying the silencing bias voltage derived from the signal-translating channel of the receiver.

As used herein, the term "limiter-amplier refers to an amplifier which is operated beyond both an upper and a lower cutoff point, so that signal inputs of all amplitudes in excess of a predetermined value transmitted by the amplifier develop output signals of constant amplitude; for example, the term may refer to a vacuum-tube amplifier adapted to operate over its entire grid-voltage plate current characteristic curve between negative grid-bias cutoi and the grid current point.

For a better understanding of the invention, together with other and further objects thereof, reference is had to the following description taken vin connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

In the drawing, Figs. 1 and 2 are circuit diagrams, partly schematic, of complete superheterodyne receivers embodying different forms of the invention.

Referring now more particularly to Fig. 1 of the drawing, there is shown schematically a complete superheterodyne radio receiver of a conventional design embodying the present invention in a preferred form. In general, the receiver includes a radio-frequency amplifier I0 having its input circuit connected to an antenna I I and ground I2 and its .output circuit connected to an oscillator-modulator or frequency changer I3. Connected in cascade with frequency changer I3, in the order named, are an intermediatefrequency amplifler I4 of one or more stages and a duplex diode-triode vacuum tube I5, the diode section of which is the detector of the receiver and the triode sectiz-n of which is the first audiofrequency amplifier of the receiver. The detector section of tube I5 is coupled to the output circuit of intermediate-frequency amplier I4 through a double-tuned intermediate-frequency selector I8--I9-2Il--2L A load circuit is provided for thV detector comprising resistors 22 and 23 `my-passed for carrier-frequency currents by condenserfJ; 24 r`nd 21. Cathode-biasing resistors 25 and 26 are provided for vacuum tube I5. Audio-frequency potentials are derived from an adjustable voltage divider across resistor 23, comprising blocking condenser 28, adjustable tapped resistor 29, resistor 30, and by-pass condenser 46, by Way of a volume control tap 3l on resistor 29, and are applied to the grid of the triode section of the tube I5 in a conventional manner. It will be understood that the various circuits just described may be of conventional construction and operation, the details of which are well known in the art, rendering further description thereof unnecessary. Considering briefly the operation of the receiver as a whole and neglecting for the moment the silencing network of this invention presently to be described, a desired received signal intercepted by antenna II is selected and amplified by radio-frequency amplifier I0, is converted to a modulated intermediatefrequency carrier in frequency changer I3, is amplified and selected by intermediate-frequency amplifier I4, and rectified by the diode section of tube I5, thereby deriving the audio frequencies of modulation and the automatic amplification control bias. The audio-frequencies of modulation are, in turn, amplified by the triode section of tube I5 and by audio-frequency amplifier I6 and are reproduced by sound reproducer I1.

Referring now more particularly to the parts of the system involving the present invention, there is provided a means for developing an automatic amplification control bias for the receiver, for operatively exciting a cathode-rayI tuning indicator or tuning eye 36, and for developing a voltage for silencing the receiver during tuning thereof or when mistuned. A voltage derived from the intermediate-frequency channel of the receiver is amplified to a constant value by limiter-amplifier 31 having its control grid coupled to the output circuit of intermediatefrequency amplifier I4 through condenser 38. An automatic amplication control bias derived from grid leak 38' is applied to the control electrodes of one or more -oi. the tubes of radio-frequency amplier I0, oscillator-modulator I3, and intermediate-frequency amplifier I4 to maintain the signal input to the detector within a relatively narrow range for a wide range of received signals. A cathode-biasing resistor 38, lay-passed by condenser 46, is provided for limiter-amplifier 31. The output circuit of limiter-amplifier 31 is coupled through asharply tuned loosely-coupled transformer 4I, 42 to a diode section in limiteramplier 31 provided with a load circuit comprising resistor 43 by-passed by condenser 44, across which a unidirectional voltage is developed. The unidirectional voltage across resistor 43 is applied to the input electrode of a cathode-ray tuning eye tube 36 through a filter comprising a series resistor 45 and a shunt condenser 46, the anode of tube 36 being connected directly to the common terminal of resistors 29 and 30 in the audio-frequency load circuit of detector I5. Auxiliary anode 41 is the target of tuning eye tube 36. Suitable operating potentials are provided in a conventional manner for the various tubes of the receiver through the terminals indicated as |Sc and +B.

In considering the operation of the arrangement just described, it will iirst be assumed that no signals of an appreciable amplitude are being received. Under these conditions the control grid of tuning-indicator tube 36 is substantially at ground potential, the resistance of the output circuit of the tuning-indicator tube is low, and a wide angle shadow is developed on the target of indicator tube 36 to show that the receiver is not tuned to a sending station. Under the conditions assumed, the conductance of tube 36 is high resulting in an appreciable voltage drop across resistor v36. It will be seen that this voltage across resistor 30 is effectively a bias upon the control grid of the triode section of the duplex tube I5 and that, under the conditions assumed, the triode section is effectively cut off and the receiver is silent. When the receiver is tuned to a signal of any usable strength, the intermediatefrequency carrier of the receiver is amplified by limiter-amplifier 31 and rectified in the diode section thereof to provide a control potential across load resistor 43 for the input circuit of tuning-indicator tube 36. For intermediate-frequency signals of any usable intensity, a relatively constant negative bias is thus applied to the control electrode of indicator tube 36 reducing the shadow angle of the tube to indicate that the receiver is correctly tuned and reducing the anode conductance of tube 36 to a minimum. Under this condition, the voltage drop across resistor 30, due to the output current of tube 36, is small and a normal operating bias is provided for the grid of duplex tube I5. Furthermore, it will be seen that, due to the delay bias developed across resistor 39, only the peaks of the voltage waves supplied by the selective circuit 4I, 42 are effective to develop a voltage across resistor 43, thus causing the silencing means to be very sensitive to receiver mistuning. It will be understood that resistor 39 can be replaced by a separate source of bias potential without essentially altering the operation o! the silencing means. The voltage developed across resistor 33 serves as a source of automatic amplification control potential and serves to maintain the intermediate-frequency signal input to the detector and to the limiter-amplifier 31 within a relai tively narrow .range for a wide variation of received signal intensities, while the limiter-amplier 31 is effective to derive an intermediate-frequency signal which vis substantially free from residual variations in amplitude for rectification by the diode section of tube 31 to develop a silencing bias voltage.

It will be seen that the network just described provides a silencing potential for the receiver, there being' substantially no reduction in the sensitivity of the receiver to signal carriers during the operation thereof. Furthermore, it will be seen that the circuit is very sensitive to mistuning because of the operation of limiter-amplier 31 and since it is responsive only to the peaks of the intermediate-frequency signals.

The embodiment of the invention shown in Fig. 2 is also utilized in a modulated-carrier signal receiver of the superheterodyne type and elements which are identical in the two figures have been given the same reference numerals. In the embodiment of the invention disclosed in Fig. 2, the signal detector of the receiver comprises cathode 60 and diode-anode 6I oi a duplex diodepentode tube 62. Audio-frequency potentials are developed across series load resistor 63, by-passed for carrier-frequency currents by condenser 64, and are applied through a coupling condenser 65 to the control electrode of the pentode section of tube 62 utilized as an audio-frequency amplifier. The audio-frequency output of the pentode section of tube 62 is applied through condenser 66 to the control electrode of audiofrequency ampliiier 61, the output circuit of audio-frequency amplifier 61 being coupled to loud-speaker I1 through a transformer 68, 69..

Automatic amplification control potentials for the receiver are developed by means of a separate A. V. C. rectifier circuit comprising a cathode 6I), diode-anode 10, and a shunt load circuit including resistors 1l, 12, 13. Automatic amplification control potentials derived from load circuit 1|, 12, 13 are applied over a circuit comprising resistor 14 to one or more of the tubes of amplier i0, oscillator-modulator I3, and intermediate-frequency amplier I4. A bias potential is provided for the control electrode of the pentode section of tube 62 through a circuit comprising resistors 15 and 13, resistor 13 being connected by way of resistors 12 and 16 to a suitable source of grid-bias potential indicated as -B in the drawing. Resistor 11 connected to source -B provides a bias potential for audio-frequency amplifier 61.

The operation of the portion of the receiver of Fig. 2 described above, which is conventional, will be apparent from the description which has been given with reference to the operation of the receiver oi Fig. 1.

Coming now to the parts of the circuit of Fig. 2 comprising the present invention, the tuning eye tube 36 is connected in such manner as to amplify the unidirectional voltage across the load resistors of the automatic volume control Aing resistors 8| and 32.

source o! bias potential, the input circuit of the indicator tube 36 being connected to resistor 14 through an audio-frequency filter comprising a series resistor 18 and a shunt condenser 19. The output circuit of indicator tube 36 is energized from the source +B through a load resistor 80 provided with an adjustable tap which is connected to the screen grid of the pentode section of tube 62 by way of a voltage divider compris- Resistor 82, connected between the screen grid of the pentode section of tube 62 and the negative terminal of the direct current supply -B, serves to stabilize the operation of the pentode section of tube 62 and to permit a more complete silencing action. An intermediate-frequency by-pass condenser 83 is coupled between the anode of the pentode section of tube 62 and ground, while a load resistor 84 is connected between its anodeiand the Source of direct current supply +B.

In considering the operation of the portion of the circuit of Fig. 2 comprising the present invention, it will first be assumed that no signals of an appreciable amplitude are being received. Under these conditions, as in the case of the circuit of Fig. 1, the control grid of the tuning indicator 36 is substantially at ground potential and the resistance of the output circuit of the indicator tube 36 is low, giving a wide angle shadow on its target 41 to show that the receiver is not tuned to a sending station. Under the conditions assumed, the anode conductanceof tube 36 is high, resulting in an appreciable voltage drop across resistor 80. The voltage drop across the portion of resistor 80 between its tap and the source of direct current supply effectively reduces the voltage which is normally supplied to the screen grid of the pentode section of tube 62 which reduction effectively cuts oi the tube and silences the receiver. When the receiver is tuned to a signal of any usable strength, the bias voltage developed in the automatic volume control system and applied to the control grid of tuning-indicator tube 36 reduces its anode conductance to a very low value. Under these conditions there is no appreciable voltage drop across resistor 80 due to the anode conductance of tube 36 and the screen potential of tube 62 is at its normal value and the received signals are reproduced by the loud-speaker of the receiver in a normal fashion.

It wil be understood that the present invention can be utilized without the tuning indicator feature illustrated in each of the gures, a vacuum-tube amplifier of conventional type being utilized in place of tuning-indicator tube 36.

It will be understood that the current or voltage developed in the output circuit of tube 36. or preferably of a conventional ampliiier tube used in place of tuning-indicator tube 36, as mentioned in the preceding paragraph, in either of the specic embodiments shown, can be utilized to operate any suitable relay in the signal-translating channel to silence the receiver. The use of an electromagnetic relay in the silencing circuit has the advantages that its operation is instantaneous during receiver tuning and, furthermore, that its characteristics can be so adjusted that the relay will not operate if the receiver is quickly tuned between two stations remote on the dial, the receiver being silent for all intermediate stations.

As illustrative of a specific embodiment of the invention, the following circuit constants are given for the embodiment of the invention shown in Fig. 2:

While there have been described what are at present considered to be the preferred embodiments of the invention, it will be understood that various changes and modiiications may be made therein without departing from the invention, and it is contemplated in the appended claims to cover all such changes and modifications as fall Within the true spirit and scope of the invention` What is claimed is:

l. A modulated-carrier signal receiver including a cathode-ray tuning indicator comprising a cathode, an anode, a control electrode and a target, an automatic ampliiication control system for said receiver for maintaining the signal amplitude at a given point of said receiver within relatively narrow limits, means comprising a limiter and rectifier including a sh arply selective circuit coupled to said point for deriving therefrom a potential substantially free from residual variations in amplitude and applying it to said control electrode, a load circuit for said indicator, and means for deriving a voltage from said load circuit to silence said receiver.

2. A modulated-carrier signal receiver of the superheterodynetypeincluding'an automatic amplification control system for maintaining the signal amplitude at a given point in the intermediate-frequency channel of said receiver within relatively narrow limits, means comprising a limitsupply voltage er-amplifler coupled to said point for deriving a signal substantially tree from residual variations in amplitude from the intermediate-frequency channel of said receiver, said limiter-ampliiier including a sharply selective circuit tuned to the intermediate frequency and following the point at which Hunting occurs, and means comprising a rectiiler coupled to said limiter-amplifier for deriving a potential to silence said receiver.

3. A modulated-carrier signal receiver of the superheterodyne type including an automatic amplification control system for maintaining the signal amplitude at a given point in the intermediate-frequency channel of said receiver within relatively narrow limits, means comprising a limiter-amplier coupled to said point for deriving a signal from the intermediate-frequency channel of said receiver substantially free from residual variations in amplitude, said limiterampliiier including a sharply selective circuit tuned to the frequency of said intermediate-frequency channel and following the point at which limiting occurs, and means comprising a rectifler responsive only to the peaks of said derived signal for developing a control voltage to silence said receiver.

4. A modulated-carrier signal receiver of the superheterodyne type including an automatic amplication control system for maintaining the signal amplitude at a given point in the intermediate-frequency channel of said receiver witllin relatively narrow limits, means comprising a limiter-amplifier coupled to said point for deriving a signal from the intermediate-frequency channel of said receiver substantially free from residual variations in amplitude, said limiter-amplier including a sharply selective circuit tuned to the frequency of said intermediate-frequency channel and following the point at which limiting occurs, means comprising a rectifier responsive only to the peaks of said derived signal for developing a-control voltage, and a direct current amplifier tube comprising'a cathode, an anode and a control electrode, means for apply- *ing said control voltage to said control electrode,

and means responsive to the anode current of said direct current amplifier tube for silencing said receiver when said control voltage is below a predetermined value.

NELSON P. CASE. 

